<b><i>Background:</i></b> Lung function impairment persists in some patients for months after acute coronavirus disease 2019 (COVID-19). Long-term lung function, radiological features, and their association remain to be clarified. Objectives: We aimed to prospectively investigate lung function and radiological abnormalities over 12 months after severe and non-severe COVID-19. <b><i>Methods:</i></b> 584 patients were included in the Swiss COVID-19 lung study. We assessed lung function at 3, 6, and 12 months after acute COVID-19 and compared chest computed tomography (CT) imaging to lung functional abnormalities. <b><i>Results:</i></b> At 12 months, diffusion capacity for carbon monoxide (DLCO_corr) was lower after severe COVID-19 compared to non-severe COVID-19 (74.9% vs. 85.2% predicted, <i>p</i> &#x3c; 0.001). Similarly, minimal oxygen saturation on 6-min walk test and total lung capacity were lower after severe COVID-19 (89.6% vs. 92.2%, <i>p</i> = 0.004, respectively, 88.2% vs. 95.1% predicted, <i>p</i> = 0.011). The difference for forced vital capacity (91.6% vs. 96.3% predicted, <i>p</i> = 0.082) was not statistically significant. Between 3 and 12 months, lung function improved in both groups and differences in DLCO between non-severe and severe COVID-19 patients decreased. In patients with chest CT scans at 12 months, we observed a correlation between radiological abnormalities and reduced lung function. While the overall extent of radiological abnormalities diminished over time, the frequency of mosaic attenuation and curvilinear patterns increased. <b><i>Conclusions:</i></b> In this prospective cohort study, patients who had severe COVID-19 had diminished lung function over the first year compared to those after non-severe COVID-19, albeit with a greater extent of recovery in the severe disease group.